The present invention relates to a temperature measuring device, and particularly to such a device utilizing a temperature sensor made of a material wherein, in an edge portion thereof, the light absorbing spectrum varies with temperature. Such measuring devices are known, for example, from Japanese Laid Open Patent Applications Nos. 8878/1980 and 6731/1982.
A typical example of a conventional temperature measuring device of this type is shown schematically in FIG. 1, in which reference numeral 1 depicts a drive circuit for a light-emitting element used as a light source 6, 2 and 4 optical fibers, 6 a light-emitting element used as a light source, and 9 a temperature sensor made of a crystalline semiconductor material or amorphous semiconductor material such as GaAs for which the higher end of its light absorbing wavelength range, and hence its light transmittance, varies with temperature. The temperature sensor 9 is disposed between the optical fibers 2 and 4 and bonded thereto by a suitable adhesive.
An example of the temperature dependency of the light transmittance of the temperature sensor 9 is shown in FIG. 2 with the wavelength plotted on the abscissa, from which it is clear that the higher end of the light absorbing wavelength range of the temperature sensor 9 is shifted to the side of longer wavelengths with an increase of temperature.
FIG. 3 is a graph showing the spectrum of light produced by the light-emitting element 6 and the spectrum of light which passes through the temperature sensor 9.
In FIG. 1, the temperature measuring device further includes a diffraction grating 15 used as a spectroscope, a photodiode array 16, comparators 17, each connected to a different photodiode of the array, and a processing circuit 18.
In operation, the light-emitting element 6 is driven by the drive circuit 1 and emits light. Assuming that the light-emitting element 6 is an LED, the spectral distribution of the emitted light follows a normal distribution, as shown in FIG. 3. The temperature sensor 9 is selected such that the higher end of its light absorbing wavelength range is positioned within the normal distribution of the spectrum of light emitted from the LED 6. Since the higher end shifts with temperature variation as shown in FIG. 2, the spectrum of light passed through the temperature sensor 9 at a certain temperature is as shown by a hatched portion in FIG. 3. This spectrum is transmitted through the optical fiber 4 to the diffraction grating 15 where it is decomposed to wavelength components which are received by respective ones of the photodiodes of the array 16. Electric signals from the photodiodes are compared by the comparators 17 associated therewith with respective comparison levels, and outputs of the comparators 17 are processed by the processing circuit 18 to obtain the position of a photodiode of the array 16 corresponding to the shortest wavelength of the light passed through the temperature sensor 9.
In such a conventional temperature measuring device, since the curve defining the higher end of absorption wavelength range of the temperature sensor 9 is not completely linear, there may occur measuring errors when there is a large intensity variation or a center wavelength variation of light received by the sensor 9 due to temperature variations of the sensor 9 or time variations of the characterstics thereof.